Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1xRTT networks” (or “1x networks” for short), which stands for “Single Carrier Radio Transmission Technology.” Another CDMA protocol that may be used is known as Evolution Data Optimized (EV-DO), perhaps in conformance with one or more industry specifications such as IS-856, Rel. 0 and IS-856, Rev. A. Other protocols may be used as well, such as Global System for Mobile Communications (GSM), Time Division Multiple Access (TDMA), WiMax, and/or any others.
These networks typically provide services such as voice, Short Message Service (SMS) messaging, and packet-data communication, among others, and typically include a plurality of base stations, each of which provide one or more coverage areas, such as cells and sectors. These base stations are typically not associated with any subscriber or small group of subscribers in particular; rather, they are placed in publicly-accessible locations and are used by the service provider's customers generally, and their coverage areas collectively blanket cities, rural areas, etc. When a mobile station is positioned in one of these coverage areas, it can communicate over the air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access.
Mobile stations and base stations conduct communication sessions (e.g. voice calls and data sessions) over frequencies known as carriers, each of which may actually be a pair of frequencies, with the base station transmitting to the mobile station on one of the frequencies, and the mobile station transmitting to the base station on the other. This is known as frequency division duplex (FDD). The base-station-to-mobile-station link is known as the forward link, while the mobile-station-to-base-station link is known as the reverse link.
Furthermore, using a sector as an example of a coverage area, base stations may provide service in a given sector on one carrier, or on more than one. An instance of a particular carrier in a particular sector is referred to herein as a sector-carrier. In a typical CDMA system, using a configuration known as radio configuration 3 (RC3), a base station can, on a given sector-carrier, transmit forward-link data on a maximum of 64 distinct channels at any time, each corresponding to a unique 64-bit code known as a Walsh code. Of these channels, typically, 61 of them are available as traffic channels (for user data), while the other 3 are reserved for administrative channels known as the pilot, paging, and sync channels.
When a base station instructs a mobile station operating on a given sector-carrier to use a particular traffic channel for a communication session, the base station does so by instructing the mobile station to tune to one of the 61 traffic channels on that sector-carrier. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. And, in addition to that forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station.
When a mobile station seeks to, as examples, originate a communication session (e.g. a voice call) or respond to a page message from a base station, the mobile station sends one or more messages known as access probes to the base station over a reverse-link access channel. As part of this process, the mobile station determines (e.g. computes) a power level at which to send the initial access probe. The mobile station then sends the initial access probe at that initial transmit power level.
In current implementations, mobile stations determine the initial transmit power (IP) by summing a set of values, all of which pertain to forward-link conditions and pilot-signal strength. In particular, when preparing to send an access probe, a mobile station may sum (i) a value that reflects the power at which the mobile station is receiving transmissions from the base station on the forward link, (ii) one or more constants, and (iii) an interference-correction factor that is derived from the signal-to-noise ratio at which the mobile station is currently receiving a pilot signal from the base station.
If the base station does not acknowledge the initial access probe, the mobile station typically sends a second access probe at an incrementally higher power level (e.g. 3 dB higher than the previous access probe). The mobile station repeats this process (i.e. incrementally increasing the power level) until either receiving an acknowledgement from the base station or reaching a set number (e.g. five) of transmitted access probes. The mobile station may then wait a timeout period, and start again at the initial power level. The mobile station may repeat this entire cycle a set number (e.g. three) of times before concluding that the base station is not reachable, or perhaps waiting a longer timeout period before starting the entire sequence over. And other variations on this access-probe-sending sequence exist as well, as this description is merely an example.